Synthesis and properties of 1,3,4-oxadiazole-containing bismaleimides with asymmetric structure and the copolymerized systems thereof with 4,4′-bismaleimidodiphenylmethane

Article abstract of DOI:10.1039/c3ra45313h

Two novel bismaleimide monomers containing 1,3,4-oxadiazole and asymmetric structure, i.e., 2-[4-(4-maleimidophenoxy)phenyl]-5-(4-maleimidophenyl)-1,3,4- oxadiazole (p-Mioxd) and 2-[3-(4-maleimidophenoxy)phenyl]-5-(4-maleimidophenyl)- 1,3,4-oxadiazole (m-Mioxd), were designed and synthesized. The chemical structures of the monomers were confirmed using Fourier transform infrared spectroscopy (FTIR), ¹H NMR and ¹³C NMR spectroscopy and elemental analysis. The thermal properties of the monomers were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The results indicate that the incorporation of the 1,3,4-oxadiazole and asymmetric structure could improve the solubility and processability of the BMI monomers and the thermal stability of the resins. Composites composed of glass cloth and 4,4′-bismaleimidodiphenylmethane (BMDM), which were modified with 2.5, 5 and 10 wt% p-Mioxd and m-Mioxd, respectively, were also prepared. The TGA and DMA results demonstrate that the resulting composites have excellent thermal stability with high residual weight percentage at 700 °C (>45%) and T_g (>450 °C).

Full text of DOI:10.1039/c3ra45313h

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Synthesis and properties of 1,3,4-oxadiazole-
containing bismaleimides with asymmetric
Cite this: RSC Adv., 2014, 4, 4646
structure and the copolymerized systems thereof
0
with 4,4 -bismaleimidodiphenylmethane
a
a
b
ab
Lianlian Xia, Xuejiao Zhai, Xuhai Xiong and Ping Chen*
Two novel bismaleimide monomers containing 1,3,4-oxadiazole and asymmetric structure, i.e., 2-[4-
4-maleimidophenoxy)phenyl]-5-(4-maleimidophenyl)-1,3,4-oxadiazole (p-Mioxd) and 2-[3-(4-
(
maleimidophenoxy)phenyl]-5-(4-maleimidophenyl)-1,3,4-oxadiazole (m-Mioxd), were designed and
synthesized. The chemical structures of the monomers were confirmed using Fourier transform infrared
1
13
spectroscopy (FTIR), H NMR and C NMR spectroscopy and elemental analysis. The thermal properties
of the monomers were investigated by differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA). The results indicate that the incorporation of the 1,3,4-oxadiazole and asymmetric
structure could improve the solubility and processability of the BMI monomers and the thermal stability
Received 24th September 2013
Accepted 31st October 2013
0
of the resins. Composites composed of glass cloth and 4,4 -bismaleimidodiphenylmethane (BMDM),
which were modified with 2.5, 5 and 10 wt% p-Mioxd and m-Mioxd, respectively, were also prepared.
DOI: 10.1039/c3ra45313h
The TGA and DMA results demonstrate that the resulting composites have excellent thermal stability
ꢀ
ꢀ
www.rsc.org/advances
with high residual weight percentage at 700 C (>45%) and T (>450 C).
g
a class of chemically resistant and thermally stable heterocyclic
polymers with high glass transition temperatures (T ) and
). Many kinds of thermally stable
Introduction
g
25
Bismaleimides and thermoplastic polyimides are high-perfor- melting temperatures (T
m
mance polymers for applications in elevated temperature polymers based on 1,3,4-oxadiazoles have been synthesized and
2
1,22
23a,b
operations for which conventional epoxy systems may not be studied, such as polyamides,
poly(arylene ether)s,
and
1–3
24
sufficiently stable. The ber-reinforced composites based on polyimides. Incorporating a 1,3,4-oxadiazole ring into the BMI
BMI resins are widely employed in the aerospace industry and skeletons in order to improve their properties without sacri-
for electronic/electrical materials such as multilayer and prin- cing their mechanical, electrical or thermal performance has
ted circuit boards and insulators, due to their excellent thermal received considerable attention during the last few decades.
4–7
and mechanical properties. Nonetheless, the ordinary BMIs Several BMI monomers containing a 1,3,4-oxadiazole moiety
4
b,16b,25
with their rigid and symmetric molecular structure suffer from have been reported in the literature.
high melting temperature and poor solubility in common monomers containing a 1,3,4-oxadiazole moiety had high
solvents, and the cured resins have inherent brittleness due to melting temperatures (T ), and sharp curing exothermic peaks
followed, which resulted in poor processability. Among them, a
Various methods to enhance the toughness of BMI resins BMI monomer with an asymmetric structure had been synthe-
However, those BMI
m
4
a
their high crosslinking density.
8–12
have been reported in the literature.
Among them, an effec- sized in order to place the terminal double bonds of BMI in
tive method is modifying the chemical structure of the BMI different chemical surroundings and improve the proc-
4b
monomers, and many efforts have been made to design and essability.
Unfortunately, it just broadened the curing
synthesize a number of new chain-extended BMI monomers, exothermic peak and the process window was still narrow.
1
3–15
16a,17
such as those containing phosphorus,
silicon,
naph-
In this paper, we reported the synthesis and characterization
of two novel BMI monomers with asymmetric structure and a
18a–c
19
18d
thalene,
biphenylene, dicyclopentadiene or dipentene,
and uorenyl cardo groups. Aromatic poly(1,3,4-oxadiazole) is 1,3,4-oxadiazole moiety. The solubility and thermal properties
20
of the monomers were investigated. The composites composed
0
of glass cloth and 4,4 -bismaleimidodiphenylmethane (BMDM),
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian which were modied with 2.5, 5 and 10 wt% p-Mioxd and m-
a
University of Technology, Dalian 116024, China. E-mail: chenping_898@126.com
Mioxd, respectively, were also prepared. Their thermal and
mechanical properties were studied using TGA and DMA.
b
Liaoning Key Laboratory of Advanced Polymer Matrix Composites Manufacturing
Technology, Shenyang Aeronautics University, Shenyang 110136, China
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Results and discussion
Synthesis and characterization of BMI monomers
The BMI monomers containing 1,3,4-oxadiazole (i.e., p-Mioxd
and m-Mioxd) were designed and synthesized following the
route illustrated in Scheme 1. C-1 was synthesized according to
25
the method reported previously. C-2a and C-2b were achieved
from the nucleophilic substitution reaction of p-chloroni-
trobenzene with p-Cresol and m-Cresol, respectively, which were
ꢁ
oxidized by a KMnO /OH system to yield C-3a and C-3b with a
4
total yield above 85%. The dinitro compounds (C-4a and C-4b)
were prepared by cyclodehydration of C-1 with C-3a and C-3b,
respectively, in the presence of PPA, and then reduced to
3
diamine compounds by hydrazine with FeCl /C as a catalyst.
The resultant diamines (p-DA and m-DA) reacted with maleic
anhydride in acetone at ambient temperature to afford the
corresponding bimaleamic acids, which were subjected to
imidization in the presence of triethylamine, sodium acetate
Fig. 1 The FTIR spectra of the diamine compounds.
Fig. 3 shows the typical FTIR for two BMI monomers (p-
and acetic anhydride to yield p-Mioxd and m-Mioxd at an overall Mioxd and m-Mioxd), the absorption peaks, i.e. 1719 (nC]O of
yield of 54.6% and 52.9%.
maleimide), 1608 (nC]N of 1,3,4-oxadiazole), 1397, 1150 (nC–N–C
The chemical structures of p-Mioxd and m-Mioxd were of maleimide) and 691 (nC]C of maleimide) conrm the struc-
1
13
16b
1
conrmed by FTIR, H NMR, C NMR and elemental analysis. ture of the two bismaleimides. The H NMR spectra of the two
In the FTIR spectrum of the diamine compounds (p-DA and m- monomers with the assignments of all the protons are pre-
DA), there are both four absorption peaks in the 3200–3500 sented in Fig. 4; there are both two sharp singlet signals at 7.25,
ꢁ
1
cm range as shown in Fig. 1, which is not consistent with the 7.20 ppm and 7.23, 7.20 ppm, which are assigned to the olenic
conventional diamines. Meanwhile, in the H NMR spectrum of protons of two maleimide end groups. This conrms the C]C
1
p-DA and m-DA, different chemical shi values (5.94 and bonds of the two maleimide end groups may have different
5
.08 ppm) and (5.96 and 5.08 ppm) for amino groups of p-DA reactivities. The integral of each proton agrees well with the
1
3
and m-DA are observed, respectively (Fig. 2). This phenomenon proposed structure. Fig. 5 shows the C NMR spectra of the two
may be attributed to the asymmetric molecular structure, which monomers; there are 18 and 20 singlet signals for p-Mioxd and
leads to the different reactivity of the two terminal amino m-Mioxd, respectively, in accordance with the number of non-
groups in the diamines, i.e., p-DA and m-DA.
equivalent carbon atoms.
Scheme 1 Synthetic route of the BMI monomers, e.g. p-Mioxd and m-Mioxd.
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The solubility and thermal properties of BMI monomers
The solubilities of p-Mioxd and m-Mioxd were evaluated in
various solvents, and the results are listed in Table 1. Both
p-Mioxd and m-Mioxd are soluble in a variety of organic
solvents, such as chloroform, dichloromethane, tetrahydrofuran,
dimethyl-sulfoxide, N,N-dimethylformamide, and N-methyl-2-
pyrrolidone. However, p-Mioxd exhibits poorer solubility in
comparison with m-Mioxd. The presumed explanation is that
the meta-substitution in m-Mioxd lowers the symmetry of the
molecular chains, thus reducing the crystallinity and improving
the solubility.
The curing behavior of p-Mioxd and m-Mioxd was investi-
ꢁ1
gated by DSC at 10 K min . As shown in Fig. 6, p-Mioxd exhibits
ꢀ
a peculiar curve showing two endothermic peaks at 174.5 C
ꢀ
ꢀ
and 247.1 C, and two exothermic peaks at 180.4 C and
2
1
Fig. 2 The H NMR spectra of the diamine compounds.
ꢀ
71.3 C, which is different from common BMI monomers,
which exhibit only one endothermic peak due to melting, and
one exothermic peak assigned to the thermally curing process.
A similar phenomenon had been reported previously, and it was
thought that the rst and the second exothermic transitions
should be correlated to the polymerization reaction between
16b
a
olenic bonds. But the E values of the rst of p-Mioxd were
ꢁ
1
ꢁ1
calculated to be 50.96 kJ mol and 49.54 kJ mol , according to
the Kissinger equation and the Ozawa equation, which were
lower than the other reported BMI monomers (about 100 kJ
ꢁ
1
18a
mol ). In comparison, the p-Mioxd sample was rst heated
at a rate of 10 K min to 200 C, kept for 30 min, and cooled
down to 150 C, then heated to 350 C at the same rate. The
obtained DSC curve was shown in Fig. 6, where we could only
observe the second exothermic transitions. To investigate
further, the m-Mioxd, p-Mioxd and the p-Mioxd aer heat
ꢁ
1
ꢀ
ꢀ
ꢀ
ꢀ
treatment at 200 C for 30 min were analyzed by a polarized
optical microscope as shown in Fig. 7, where we can see only
one crystal form in (a) and (c), and two crystal forms in (b), so we
think that the rst heat transition should be ascribed to the
transformation of the crystal form. The m-Mioxd has one
Fig. 3 The FTIR spectra of the BMI monomers.
ꢀ
endothermic peak at 206.3 C attributed to melting, and one
ꢀ
exothermic peak at 283.1 C assigned to the curing process. The
values of the curing characteristics for p-Mioxd and m-Mioxd
were reported in Table 2. Compared with p-Mioxd undergoing a
rapid thermal curing upon melting, m-Mioxd has a lower
melting point, and a broader processing window. This may be
responsible for the meta-substitution in the structure of m-
Mioxd, which lowers the symmetry of molecular chain, thus
reducing the crystallinity and melting temperature.
The thermal properties of p-Mioxd and m-Mioxd were
ꢁ1
investigated by TGA at 20 K min in nitrogen. As shown in
Fig. 8, both display excellent thermal stability with tempera-
ꢀ
ꢀ
tures for 5% weight loss at 504.2 C and 498.7 C. The residual
ꢀ
weights of p-Mioxd and m-Mioxd at 700 C are both above 60%.
The activation energies (E
a
) of the curing reaction of p-Mioxd
and m-Mioxd were calculated from DSC measurement by
1b,26a
employing the Kissinger equation and the Ozawa equation.
Kissinger equation:
h ꢀ
ꢁi.
ꢂ
ꢃ
2
1
d ln b=T
d 1=T
a
¼ ꢁE =R
p
p
Fig. 4 The H NMR spectra of the BMI monomers.
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13
Fig. 5 The C NMR spectra of the BMI monomers.
Ozawa equation:
ꢂ
ꢃ
dðlog bÞ=d 1=T
p
¼ ꢁ0:4567E
a
=R
where E
a
is the activation energy, R is the gas constant, b is the
heating rate, and T
p
is the exothermic transition temperature of
the corresponding b. To estimate the activation energy (E ), a
a
series of dynamic DSC scans were conducted at heating rates of
ꢁ
1
2
.5, 5, 10, 15 and 20 K min , and the results were shown in
Table 3. On the basis of the Kissinger equation and the Ozawa
equation, the kinetic plots were drawn in Fig. 9 and the E
a
2
values were obtained from the slope of the graph of ln(b/T
versus 1/T and of ln b versus 1/T . The E values of the rst heat
transition of p-Mioxd were calculated to be 50.96 kJ mol and
p
)
p
p
a
ꢁ
1
ꢁ
1
4
9.54 kJ mol according to the Kissinger equation and the
Ozawa equation, which were lower than the other reported BMI
ꢁ
1
18a
monomers (about 100 kJ mol ). The E values of the second
a
Fig. 6 The DSC curves of p-Mioxd and m-Mioxd at a heating rate of 10
ꢁ1
curing process of p-Mioxd and m-Mioxd, i.e. 124.7 kJ mol
29.0 kJ mol and 125.4 kJ mol , 128.7 kJ mol , respectively,
were very comparable with typical BMI.
,
ꢁ1
ꢁ1
K min in nitrogen, and p-Mioxd
2
: first heated at a rate of 10 K min
ꢁ1
ꢁ1
ꢁ1
1
ꢀ
ꢀ
to 200 C, kept for 30 min, cooled down to 150 C and then heated
ꢀ
again to 350 C at the same rate.
Table 1 The solubility of BMI monomers
Ethanol
Acetone
Toluene
DCM
Chloroform
THF
DMF
DMSO
NMP
p-Mioxd
m-Mioxd
ꢁ
ꢁ
+
+
+
+
+
++
++
++
+
++
++
++
++
++
++
++
a
ꢁ1
‘+’ ¼ soluble (>10 mg mL ), ‘++’ ¼ soluble under heating, ‘ꢁ’ ¼ insoluble. DCM, dichloromethane; THF, tetrahydrofuran; DMF, N,N-
dimethylformamide; DMSO, dimethylsulfoxide; NMP; N-methyl-2-pyrrolidinone.
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ꢀ
Fig. 7 The polarized optical microscopy photograph of BMI monomers. (a) m-Mioxd, (b) p-Mioxd, and (c) p-Mioxd after heat treatment at 200 C
for 30 min.
Thermal and mechanical properties of composites
composited with BMI mixtures and glass cloth
Both p-Mioxd and m-Mioxd were found to be difficult for pro-
cessing due to the high melting points and narrow processing
window. Since mixtures of BMI monomers have been success-
fully employed to decrease the melting and curing temper-
26b,27
ature,
MBMI and m-Mioxd/MBMI with various percentages (2.5, 5,
0 wt%) were prepared in our study. The DSC investigation of
thus BMI monomer mixtures based on p-Mioxd/
1
the BMI monomer mixtures is shown in Fig. 10 and Table 2,
where we can see that all samples display a sharp melting
ꢀ
transition at around 160 C and a broad exothermic transition
assigned to the curing process. The T of p-Mioxd/MBMI
m
ꢀ
ꢀ
systems is found to increase from 161.5 C to 162.4 C with the
weight percentage of p-Mioxd increasing from 2.5% to 5%, but
ꢀ
decrease to 160.5 C when the percentage is 10%. T
p
repre- Fig. 8 The TGA curves of the resins based on p-Mioxd and m-Mioxd at
ꢀ
ꢁ1
sents a similar trend, which increases from 293.3 C to a heating rate of 20 K min in nitrogen.
ꢀ
ꢀ
2
93.7 C and then decreases to 292.1 C. T and T of the
m p
m-Mioxd/MBMI systems display a contrary trend compared
with the p-Mioxd/MBMI systems, which rst decrease and mixtures have a lower T
then increase with the addition of p-Mioxd. The results indi- cessing window, dened as T
cate that the processing properties of p-Mioxd and m-Mioxd
ꢀ
of about 160 C and a wider pro-
m
ꢀ
ꢁ T
, of about 90 C.
The thermal stability of the BMI mixtures with various
i
m
could be improved using mixtures of them with MBMI. All percentages were investigated by TGA (Fig. 11 and Table 4). It is
Table 2 The DSC cure characteristics of BMI monomers and their mixtures with MBMI
a
m
ꢀ
b
ꢀ
c
p
ꢀ
d
f
ꢀ
e
m
ꢀ
ꢁ1
Code
T
( C)
T
i
( C)
T
( C)
T
( C)
T
i
ꢁ T
( C)
DH (J g )
BMI monomers
MBMI
p-Mioxd
m-Mioxd
160.3
247.1
206.3
174.5
177.8
248.3
237.9
—
219.5
271.3
283.1
180.4
245.7
306.2
308.5
—
17.5
—
31.6
—
60.9
189.3
109.1
6.8
p-Mioxd
2
p-Mioxd/MBMI (wt%)
2
5
1
.5%
%
0%
p-Mioxd-2.5
p-Mioxd-5
p-Mioxd-10
161.7
162.4
160.5
243.6
253.2
248.3
293.3
293.7
292.1
335.9
332.7
332.5
88.1
90.8
87.8
115.8
90.3
90.8
m-Mioxd/MBMI (wt%)
2
5
1
.5%
%
0%
m-Mioxd-2.5
m-Mioxd-5
m-Mioxd-10
162.2
158.4
159.7
247.6
249.6
251.2
294.4
289.7
293.9
339.7
343.5
344.7
85.4
91.2
91.5
121.3
150.5
108.9
a
e
b
c
d
The melting transition temperature. The initial curing temperature. The exothermic transition temperature. The nal curing temperature.
The processing window.
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Table
3 The DSC analysis of p-Mioxd, m-Mioxd monomers at
different heating rates
Heating rate
ꢁ
1
a
ꢀ
b
ꢀ
c
ꢀ
b (K min
)
T
p1 ( C)
T
p2 ( C)
Tp3 ( C)
2
5
1
1
2
.5
174.6
177.9
180.4
182.1
182.9
251.2
262.1
276.3
288.1
296.8
251.8
264.4
283.1
290.4
296.8
0
5
0
a
b
Fig. 10 The DSC curves of the BMI monomer mixtures at a heating
The rst exothermic transition temperature of p-Mioxd. The second
c
ꢁ1
exothermic transition temperature of p-Mioxd. The exothermic rate of 10 K min in nitrogen. (a) p-Mioxd/MBMI systems, (b) m-
transition temperature of m-Mioxd.
Mioxd/MBMI systems.
observed that the temperatures required for 5% and 10% weight
loss of the p-Mioxd/MBMI systems are found to increase from
contrary trend, decreasing from 7.82 GPa to 6.95 GPa. This
unusual phenomenon may be attributed to the para-substitu-
tion in the structure of p-Mioxd, which resulted in the tight
packing of molecules. From Fig. 13(a) and (b), it is observed that
ꢀ
ꢀ
ꢀ
ꢀ
5
11.2 C to 519.4 C and from 512.7 C to 521.7 C, respectively,
ꢀ
and the residual weight percentage at 700 C apparently
increased from 48.5% to 58.3% with increasing concentration
of p-Mioxd. The temperatures required for 5% and 10% weight
the T
g
of composites composed of p-Mioxd/MBMI or m-Mioxd/
ꢀ
MBMI with glass cloth are above 450 C, except for the p-Mioxd-
ꢀ
loss of the m-Mioxd/MBMI systems oat at around 516.5 C and
10 system, indicating the excellent thermal stability of the
ꢀ
5
7
19.5 C, respectively, while the residual weight percentage at
resulting composites.
ꢀ
00 C increased from 54.8% to 62.2% with increasing
concentration of m-Mioxd, as shown in Table 4. The results
indicate that incorporation of p-Mioxd and m-Mioxd improves
the thermal stability and enhances the residual weight
Experimental
Materials
ꢀ
percentage at 700 C of pure BMI monomers, due to the thermal
stable aromatic heterocyclic and 1,3,4-oxadiazole structure of p-Nitrobenzoic acid, p-chloronitrobenzene, p-Cresol and m-
p-Mioxd and m-Mioxd.
Cresol were supplied by Sinopharm Chemical Reagent Co.
0
The thermal mechanical properties are the most important (Shanghai, China), and used as received. 4,4 -Bismaleimidodi-
properties of high-performance materials for advanced phenylmethane (BMDM) was purchased from Yantai Hengxing
composites. In our study, DMA was employed to assess these Chemical Industrial Technologic Company and puried by
column chromatography on silica gel. The glass cloth were
supplied by Anhui Danfeng Group. Acetone and triethylamine
were puried by distillation under reduced pressure over
properties of the composites composed of p-Mioxd/MBMI or
m-Mioxd/MBMI with glass cloth. Fig. 12 and 13 show the
temperature dependence of storage modulus (E ) and loss
0
tangent (tan d) for the prepared composites, respectively. As calcium hydride. Other chemical reagents were purchased from
shown in Fig. 12(a) and (b), the E values for the composites various commercial sources and used directly.
composed of modied resins in the glassy region are found to
be decreased; this may be caused by the chain extension of p-
Measurements
Mioxd and m-Mioxd compared with MBMI, which hinders the
0
tight packing of the molecular chain. The E values increase Fourier transform infrared (FTIR) spectra (KBr) were recorded
ꢁ1
from 6.06 GPa to 9.57 GPa with increasing percentage concen- on a Nicolet-20DXB IR spectrophotometer at 4 cm resolution
tration of p-Mioxd, while m-Mioxd modied resins display a and signal-averaged over 32 scans. Samples were prepared by
Fig. 9 The kinetic plots of BMI monomers based on Ozawa equation and Kissinger equation, (a) first heat transition of p-Mioxd, (b) curing
process of p-Mioxd, and (c) curing process of m-Mioxd.
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ꢁ1
Fig. 11 The TGA curves of the BMI monomer mixtures at a heating rate of 20 K min in nitrogen. (a) p-Mioxd/MBMI systems and (b) m-Mioxd/
MBMI systems.
ꢁ
1
Table 4 TGA analysis of BMI monomers and their mixtures with MBMI mg samples were used at a heating rate of 10 K min under a
ꢁ
1

ow of nitrogen (20 mL min ).
a
ꢀ
b
ꢀ
c
ꢀ
d
Sample
T
5% ( C)
T
10% ( C)
T
max ( C)
R.W. (%)
Polarized optical microscopy graphs were recorded by Nikon
OPTIPHOT2-POL.
Thermogravimetric analysis (TGA) was performed on a Per-
kin-Elmer TGA-7 thermal analyzer and all samples (around
MBMI
p-Mioxd
m-Mioxd
p-Mioxd-2.5
p-Mioxd-5
p-Mioxd-10
m-Mioxd-2.5
m-Mioxd-5
m-Mioxd-10
502.2
504.2
498.7
511.2
518.1
519.3
516.4
514.8
516.5
505.6
514.5
508.1
512.7
520.9
521.7
519.5
517.7
519.2
509.1
517.9
511.3
515.8
521.8
523.2
520.9
521.8
519.6
44.9
64.0
61.7
48.5
55.8
58.3
54.8
56.1
62.2
ꢀ
ꢁ1
6
mg) were heated from 25 to 750 C at a rate of 20 K min
,
ꢁ1
under a puried nitrogen ow rate of 60 mL min
Dynamic mechanical analysis (DMA) was done using a TA
Instruments Q800 DMA with an amplitude of 20 mm, driving
frequency of 1.0 Hz and a temperature ramp rate of 3 K min in
a nitrogen atmosphere. The specimens were cut to dimensions
.
ꢁ
1
a
b
The temperature at 5% weight loss. The temperature at 10% weight
c
loss. The temperature corresponding to the maximum rate of weight of 30 mm ꢂ 6 mm ꢂ 2 mm for the single cantilever mode.
d
ꢀ
loss. The residual weight percentage at 700 C.
Monomer synthesis
Synthesis of
dispersing the material in KBr and compressing the mixtures to
form disks.
4-nitrobenzhydrazide
(C-1).
4-Nitro-
1
13
H NMR and C NMR were recorded on a Varian INOVA benzhydrazide was synthesized according to the method
26a
ꢁ1
4
00 MHz NMR spectrometer, using tetramethylsilane (TMS) reported previously at a yield of 95%. FTIR (KBr; cm ): n ¼
1
as an internal reference. Chemical shis (d) were reported 3332, 3276 (–NH–, –NH ), 1647 (–C]O), 1506, 1340 (–NO ). H
2
2
in ppm.
NMR (400 MHz; CDCl ): d ¼ 8.30 (d, J ¼ 8.89 Hz, 2H, Ar–H), 8.05
3
Elemental analysis was recorded using an Elemental Vario (d, J ¼ 8.89 Hz, 2H, Ar–H), 10.14 (s, 1H, –NH), 4.7 (s, 2H, –NH ).
2
0
EL III instrument.
Synthesis of 4-methyl-4 -nitrodiphenyl ether (C-2a) and 3-
0
Differential scanning calorimetry (DSC) measurements were methyl-4 -nitrodiphenyl ether (C-2b). p-Cresol (27.6 mL, 0.26
conducted with an NETZSCH DSC 204 instrument. About 6–9 mol), anhydrous potassium carbonate (41.4 g, 0.3 mol), and dry
Fig. 12 The storage modulus for various composites. (a) Composites based on p-Mioxd/MBMI and (b) composites based on m-Mioxd/MBMI.
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Fig. 13 The damping factor (tan d) for various composites. (a) Composites based on p-Mioxd/MBMI and (b) composites based on m-Mioxd/
MBMI.
2
dimethyl-formamide (DMF, 150 mL) were placed into a 500 mL (–COOH), 1692 (–C]O), 1504, 1346 (–NO ), 1251, 1110 (Ar–O–Ar).
ꢀ
1
three-necked ask. The mixture was heated to 120 C and stir-
H NMR (400 MHz; d-DMSO): d ¼ 13.46 (s, 1H, –COOH), 8.28 (d,
red for 2 h, then a solution of p-chloronitrobenzene (41.8 g, 0.25 J ¼ 9.2 Hz, 2H, Ar–H), 7.86 (d, 1H, Ar–H), 7.66 (s, 1H, Ar–H), 7.63
mol) in DMF (150 mL) was added dropwise over 1 h. Aer the (t, 1H, Ar–H), 7.48 (d, 1H, Ar–H), 7.20 (d, J ¼ 9.2 Hz, 2H, Ar–H).
addition, the reaction mixture was heated for another 10 h, and
Synthesis of 2-[4-(4-nitrophenoxy)phenyl]-5-(4-nitrophenyl)-
then separated by suction ltration. The resulting yellowish 1,3,4-oxadiazole (C-4a) and 2-[3-(4-nitrophenoxy) phenyl]-5-(4-
solid was precipitated by pouring the cooled ltrate into ice-cold nitrophenyl)-1,3,4-oxadiazole (C-4b). C-1 (19.9 g, 0.11 mol), C-3a
water with constantly stirring. The crude C-2a was collected by (25.9 g, 0.1 mol), polyphosphoric acid (PPA; 400 g) were placed
ꢀ

ltration, washed thoroughly with distilled water and dried in into a ask, and then stirred to form a solution at 120 C for 6 h.
an oven. The product was puried by column chromatography Aer cooling down to room temperature, the reaction mixture
on silica gel (petroleum ether/dichloromethane), (54.37 g, yield was poured into distilled water with stirring. The solid precip-
ꢁ
1
9
1%). FTIR (KBr; cm ): n ¼ 1506, 1344 (–NO ), 1248, 1110 (Ar– itate was ltered, washed with distilled water and subsequently
2
1
O–Ar). H NMR (400 MHz; CDCl ): d ¼ 8.19 (d, J ¼ 9.2 Hz, 2H, with the dilute solution of sodium hydroxide, and nally dried
3
ꢁ1
Ar–H), 7.23 (d, J ¼ 8.3 Hz, 2H, Ar–H), 7.02–6.97 (m, 4H, Ar–H), in an oven (33.9 g, yield 84%). FTIR (KBr; cm ): n ¼ 1506, 1344
.38 (s, 3H, –CH ). (–NO ), 1250, 1114 (Ar–O–Ar), 1610 (C]N), 968 (C–O–C).
C-2b was synthesized using an analogous method as
H NMR (400 MHz; d-DMSO): d ¼ 8.47 (d, J ¼ 8.84 Hz, 2H, Ar–H),
described for C-2a, (yield: 92%). FTIR (KBr; cm ): n ¼ 2923, 8.40 (d, J ¼ 8.84 Hz, 2H, Ar–H), 8.32 (d, J ¼ 9.15 Hz, 2H,
2
3
2
1
ꢁ
1
1
2
(
1
951 (–CH
400 MHz; CDCl
H, Ar–H), 7.26 (s, 1H, Ar–H), 7.07 (d, 1H, Ar–H), 7.01 (d, J ¼ 9.10
Hz, 2H, Ar–H), 6.89 (d, 1H, Ar–H), 2.38 (s, 3H, –CH ).
3
), 1501, 1345 (–NO
): d ¼ 8.20 (d, J ¼ 9.10 Hz, 2H, Ar–H), 7.30 (t, Ar–H), 7.31 (d, J ¼ 9.15 Hz, 2H, Ar–H).
C-4b was synthesized using an analogous method as
2
), 1250, 1108 (Ar–O–Ar). H NMR Ar–H), 8.27 (d, J ¼ 8.63 Hz, 2H, Ar–H), 7.44 (d, J ¼ 8.63 Hz, 2H,
3
ꢁ
1
described for C-4a (yield: 86%). FTIR (KBr; cm ): n ¼ 1509,
3
Synthesis of 4-(4-nitrophenoxy) benzoic acid (C-3a) and 3-(4- 1347 (–NO ), 1248, 1106 (Ar–O–Ar), 1606 (C]N), 965 (C–O–C).
nitrophenoxy) benzoic acid (C-3b). C-2a (11.95 g, 0.05 mol),
sodium hydroxide (10 g, 0.25 mol), distilled water (200 mL) and 9.10 Hz, 2H, Ar–H), 8.10 (d, 1H, Ar–H), 7.97 (s, 1H, Ar–H), 7.78 (t,
2
1
H NMR (400 MHz; d-DMSO): d ¼ 8.43 (d, 4H, Ar–H), 8.30 (d, J ¼
pyridine (100 mL) were placed into a 500 mL three-necked ask. 1H, Ar–H), 7.51 (d, 1H, Ar–H), 7.26 (d, J ¼ 9.10 Hz, 2H, Ar–H).
ꢀ
The mixture was then heated to 100 C, then potassium
Synthesis of 2-[4-(4-aminophenoxy)phenyl]-5-(4-amino-
4
hypermanganate (KMnO , 39.5 g, 0.25 mol) was added three phenyl)-1,3,4-oxadiazole (p-DA) and 2-[3-(4-aminophenoxy)
times over 0.5 h. Aer the addition, the reaction mixture was phenyl]-5-(4-aminophenyl)-1,3,4-oxadiazole (m-DA). C-4a (12.1
reuxed for 9 h until the potassium hypermanganate faded, and g, 0.03 mol), activated charcoal (8 g), FeCl3 (0.5 g), and 2-
was then separated by suction ltration. The resulting white methoxyethanol (150 mL) were placed into a 250 mL three-
ꢀ
solid was precipitated by adding 20 mL concentrated sulfuric necked ask. The reaction mixture was heated to 110 C with
acid into the cooled ltrate under constant stirring. The C-3a stirring, and then 85% hydrazine monohydrate (25 mL) was
was collected by ltration, washed thoroughly with distilled added dropwise over 1 h. Aer the addition, the mixture was
ꢁ
1
water and dried in an oven (12.3 g, yield 95%). FTIR (KBr; cm ): heated for another 10 h under a nitrogen atmosphere, and then
n ¼ 2500–3300 (–COOH), 1685 (–C]O), 1507, 1347 (–NO ), 1237, separated by suction ltration. The resulting white solid was
110 (Ar–O–Ar). H NMR (400 MHz; d-DMSO): d ¼ 12.95 (s, 1H, precipitated by pouring the cooled ltrate into ice-cold water
COOH), 8.30 (d, J ¼ 8.9 Hz, 2H, Ar–H), 8.03 (d, J ¼ 8.41 Hz, 2H, under constant stirring. The precipitate was collected by ltra-
Ar–H), 7.25–7.27 (m, 4H, Ar–H). tion, dried and puried using ethanol (9.18 g, yield 89%). FTIR
C-3b was synthesized using an analogous method as (KBr; cm ): n ¼ 3437, 3372, 3323, 3214 (–NH
2
1
1
–
ꢁ
1
2
), 1236, 1067 (Ar–
described for C-3a (yield: 93%). FTIR (KBr; cm ): n ¼ 2500–3300 O–Ar), 1606 (C]N), 960 (C–O–C). H NMR (400 MHz; d-DMSO):
ꢁ1
1
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d ¼ 8.0 (d, J ¼ 8.93 Hz, 2H, Ar–H), 7.74 (d, J ¼ 8.69 Hz, 2H, Ar–H), Preparation of the composites composited with BMI mixtures
7
.03 (d, J ¼ 8.93 Hz, 2H, Ar–H), 6.84 (d, J ¼ 8.76 Hz, 2H, and glass cloth
Ar–H), 6.69 (d, J ¼ 8.69 Hz, 2H, Ar–H), 6.63 (d, J ¼ 8.76 Hz, 2H,
BMI monomer mixtures, i.e. p-Mioxd/MBMI or m-Mioxd/MBMI
Ar–H), 5.94 (s, 2H, –NH ), 5.08 (s, 2H, –NH ).
2
2
(
2.5, 5, 10 wt%) was dissolved in DMF at a concentration of 0.5 g
m-DA was synthesized using an analogous method as
ꢁ1
mL , A glass cloth was impregnated with the solution for 24 h,
ꢁ1
described for p-DA (yield: 87%). FTIR (KBr; cm ): n ¼ 3470,
383, 3329, 3220 (–NH ), 1213, 1068 (Ar–O–Ar), 1605 (C]N), 959
C–O–C). H NMR (400 MHz; d-DMSO): d ¼ 7.75 (d, J ¼ 8.7 Hz,
H, Ar–H), 7.7 (d, 1H, Ar–H), 7.53 (t, 1H, Ar–H), 7.49 (s, 1H, Ar–
H), 7.10 (d, 1H, Ar–H), 6.85 (d, J ¼ 8.8 Hz, 2H, Ar–H), 6.70 (d, J ¼
.7 Hz, 2H, Ar–H), 6.64 (d, J ¼ 8.8 Hz, 2H, Ar–H), 5.96 (s, 2H,
NH ), 5.08 (s, 2H, –NH ).
ꢀ
and then hung vertically and dried at 80 C in vacuo. The preregs
3
(
2
2
were cut into pieces with dimensions 35 mm ꢂ 6.5 mm, stacked
1
to a proper height, put between two steel sheets in a hydraulic
ꢀ
ꢀ
press and cured following the regime: 180 C for 4 h, 290 C for
ꢀ
6

h, 300 C for 2 h. The weight percentage of the resin in the
nal composites was determined to be around 40%.
8
–
2
2
Synthesis of 2-[4-(4-maleimidophenoxy)phenyl]-5-(4-mal-
eimidophenyl)-1,3,4-oxadiazole (p-Mioxd) and 2-[3-(4-mal- Conclusions
eimidophenoxy)phenyl]-5-(4-maleimido-phenyl)-1,3,4-oxadiazole
Two novel bismaleimide monomers containing 1,3,4-oxadiazole
and asymmetric structure, i.e. p-Mioxd and m-Mioxd, were
designed and synthesized, and both monomers had good
solubility in common organic solvents. DSC investigations
indicated that p-Mioxd had two exothermic transitions, which
related to the transformation of the crystal form and the
thermal curing of p-Mioxd, whereas m-Mioxd had only one
exothermic transition related to the thermal curing of m-Mioxd.
TGA investigations showed that the temperature for 5% and
(
m-Mioxd). C-5a (10.32 g, 0.03 mol), acetone (100 mL) were
placed into a 250 mL three-necked ask, and stirred at room
temperature. Thereaer a solution of maleic anhydride (6.17 g,
0.063 mol) in acetone (80 mL) was added into the reaction
mixture dropwise. The suspension turned into a clear solution
aer the addition, and was stirred for another 9 h. The N,N-
bismaleamic acid precipitated out, and was separated by
suction ltration, washed with acetone to remove remain
maleic anhydride and dried. A 250 mL three-necked ask was
charged with acetone (150 mL), N,N-bismaleamic acid (10.8 g,
ꢀ ꢀ
1
4
0% weight loss was 504.2 C, 514.5 C for p-Mioxd and
ꢀ
ꢀ
98.7 C, 508.1 C for m-Mioxd, respectively. The residual weight
0.02 mol), triethylamine (1.2 mL) and sodium acetate (0.02 g).
ꢀ
percentages at 700 C are all above 60%. The TGA results reveal
that the p-Mioxd/MBMI systems and p-Mioxd/MBMI systems
had excellent thermal stability with 10% weight loss above
The reaction mixture was heated and reuxed until the
suspension turned into a clear solution. Then acetic anhydride
(5 mL) was added dropwise, and reux was continued for an
ꢀ
ꢀ
5
4
10 C and residual weight percentage at 700 C in range
8–62%. The thermal mechanical properties of composites
additional 6 h. The resulting solid was precipitated by pouring
the reaction solution into ice-cold water with constant stirring.
The precipitate was collected by ltration, washed with sodium
bicarbonate until it was free from acetic acid, then nally
washed with water and dried in vacuo, (15.1 g, yield 89%). FTIR
composed of p-Mioxd/MBMI or m-Mioxd/MBMI with glass cloth
were obtained by DMA. The results showed a high storage
ꢀ
modulus (>6.5 GPa) and T (>450 C), indicating the excellent
g
ꢁ
1
thermal stability of the resulting composites.
(
1
KBr; cm ): n ¼ 1720 (C]O), 1611 (C]N), 1400, 1156 (C–N–C),
1
266, 1072 (Ar–O–Ar), 956 (C–O–C), 691 (C]C). H NMR (400
MHz; d-DMSO): d ¼ 8.25 (d, J ¼ 8.70 Hz, 2H, Ar–H), 8.19 (d, 2H,
Acknowledgements
Ar–H), 7.64 (d, J ¼ 8.70 Hz, 2H, Ar–H), 7.43 (d, 2H, Ar–H), 7.29
The authors are grateful to the donors of the National Defense
(
d, 2H, Ar–H), 7.27 (d, 2H, Ar–H), 7.25 (s, 2H, C]C–H), 7.20 (s,
1
3
12th 5-year program Fundamental Research Program (no.
2
1
1
H, C]C–H). C NMR (100 MHz; d-DMSO): d ¼ 170.42, 170.04,
64.26, 163.97, 160.23, 154.99, 135.38, 135.18, 135.13, 129.57,
29.27, 128.23, 127.71, 127.51, 122.67, 120.39, 119.28, 118.82.
A3520110001), Liaoning Excellent Talents in University (no.
LR2013002) and the National Natural Science Foundation of
China (Grant No.51303107) for generous supporting of our
research. Many thanks to all those who were involved in this
work.
Elemental analysis: (found: C, 66.59; H, 3.15; N, 11.23.
C H O N requires C, 66.67; H, 3.17; N, 11.11%).
2
8
16 6 4
m-Mioxd was synthesized using an analogous method as
ꢁ
1
described for p-Mioxd (yield 87%). FTIR (KBr; cm ): n ¼ 1713
C]O), 1615 (C]N), 1389, 1147 (C–N–C), 1247, 1065 (Ar–O–Ar),
(
Notes and references
1
9
56 (C–O–C), 689 (C]C). H NMR (400 MHz; d-DMSO): d ¼ 8.26
d, J ¼ 8.7 Hz, 2H, Ar–H), 7.96 (d, 1H, Ar–H), 7.80 (s, 1H, Ar–H),
.69 (t, 1H, Ar–H), 7.63 (d, J ¼ 8.7 Hz, 2H, Ar–H), 7.41 (d, J ¼ 8.9
Hz, 2H, Ar–H), 7.36 (d, 1H, Ar–H), 7.26 (s, 2H, C]C–H), 7.23 (d,
(
7
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3
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1
1
35.38, 135.25, 135.16, 132.02, 129.26, 127.86, 127.75, 127.47,
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6
N
4
requires
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